1. Field of the Invention
This invention relates to an apparatus for high-energy electron diffraction (HEED) analysis of reflection type.
2. Description of the Related Art
Among conventional techniques for electron diffraction there is known an analytical technique referred to as reflection type high-energy electron diffraction (abbreviated and hereinafter also referred to as xe2x80x9cRHEEDxe2x80x9d), in which a beam of electrons is accelerated and focused for incidence at a small angle on the surface of a specimen and diffracted therein, and diffracted beams produced are reflected, forming an image.
A system for RHEED in general has a makeup as illustrated in FIG. 14.
In the RHEED system 1 as shown in FIG. 14, a beam of electrons emitted from a source thereof 2 is directed towards an anode 3. The electron beam having passed through an opening 3a formed in the anode is accelerated and focused through an electronic objective lens 4 and objective diaphragm or lens stop 5 to impinge on a specimen 6 where beam diffraction produces diffracted electron beams which are reflected to impinge on a screen 7. The intensity of the diffracted beams is detected by a CCD (charge coupled device), not shown.
Using the RHEED system 1 in a low or reduced pressure process such as molecular beam epitaxy allows a film to grow, and its growth to be monitored, at an accuracy of one atomic layer, based on the fact that the mirror reflection intensity of the electron beams oscillates reflecting the irregularity of a surface on a level of one atomic layer.
By the way, for the source of electron beams 2 use has widely been made of an emitter of thermionic emission type which is readily usable at a pressure as low as 10xe2x88x923 Pa or even lower. An electron beam having a diameter of 20 to 40 xcexcm is thereby produced.
In such a case, if the electron beam is incident at an angle of 0.5 to 5 degrees on the specimen 6 and if the electronic objective lens 4 and the specimen 6 are spaced apart by a distance that is equal to a focal distance f as long as 105 mm or more, then the electron beam from the electron beam source 2 focused on the specimen 6 by the electronic objective lens 4 cannot be reduced in minimum diameter to less than 100 xcexcm.
Thus, if the electron beam is incident at, for example, 3 degrees, on the specimen 6 it will have a cross section projected on a surface of the specimen whose diameter in its longitudinal direction is as large as about 2 mm. This means it is not possible, and this has indeed made it impossible, to monitor and identify on the specimen 6 a varying thin film structure as small as 2 mmxc3x97100 xcexcm or less.
It should be noted that this existing failure in the RHEED lies where on the other hand the state of art has the recent years seen a variety of in-vacuum film forming apparatus proposed, based on a so-called combinatorial technique that makes it possible to form on a given specimen very thin films adjacent to each other, called pixels, which vary in film forming conditions, simultaneously in a single process step. The pixels lying adjacent to each other commonly have a size of at most 100 xcexcm xc3x97100 xcexcm and typically less, which it is desirable to monitor and identify.
Further, while in the conventional applications of the RHEED the position of a spot on the specimen irradiated with the electron beam can be located enough on visual observation, this does not apply to the combinatorial synthesis used to prepare a plurality of thin films parallel to one another. Then, the need arises to adjust the position of irradiation with the electron beam, accurately in the order of the size, e.g., 100 xcexcmxc3x97100 xcexcm, of each of the pixels lying adjacent to each other so that the irradiating beam may be positioned precisely pixel by pixel.
Therefore, in order for pixels to be monitored and identified each individually with precision under their given respective irradiating conditions, a need arises to form a beam of electrons having an area of irradiation that is at most equal to the size of a pixel or less.
Mention may further be made of the fact that a RHEED system is also known provided with what is called a differential evacuation structure as shown in FIG. 15.
Such a RHEED system as illustrated in FIG. 15 differs from that shown in FIG. 14 in that a region of the system extending from the electron beam source 2 over to the electronic objective lens 4 and objective diaphragm or lens stop 5 is disposed in a low pressure chamber 9 and an area of the electron beam source 2 is reduced in pressure by a pump 9a and held at a pressure of 10xe2x88x923 Pa or less.
By reason of the limitation imposed by the structure with a single pressure stage of the objective diaphragm or lens stop 5 to make a maximum differential pressure ratio attainable at most about 1/1000, holding the region of the electron beam source 2 under a pressure of 10xe2x88x923 Pa or less has required the region of the specimen 6 to be placed under a pressure of several Pa or less.
Such structured RHEED system 8, as does the RHEED system 1 of FIG. 14, makes it impossible to monitor and identify on the specimen 6 varying thin film structures as small as 2 mmxc3x97100 xcexcm in size.
It is accordingly an object of the present invention to provide a high-energy electron diffraction analysis apparatus that is capable of monitoring and identifying varying thin film structures of 100 xcexcmxc3x97100 xcexcm or less in size synthesized parallel to one another on a specimen.
In order to achieve the objects mentioned above, the present invention provides a high-energy electron beam diffraction analysis apparatus in which a specimen is securely held in a vacuum chamber capable of evacuation to a high vacuum and is irradiated with a high-energy beam of electrons to form an image of reflected beam diffraction, which apparatus comprises: a first casing adapted to accommodate therein an electron beam source for producing an electron beam, the first casing having a first aperture; and a second casing coupled to the said first casing via the said first aperture and having an end portion formed with a second aperture coaxial with the said first aperture, wherein the first and second casings form a differential evacuation structure with the said first casing held at a lower pressure than the said second casing, and the electron beam source accommodated in the first casing comprises a field emission type electron emitter.
This construction that makes up the region from the electron beam source constituted with a field emission type electron emitter to the specimen in a two-stage differential evacuator structure allows the first casing to be held under a low pressure of 10xe2x88x926 Pa or less ideally suited for the field emission type electron emitter and yet a region of the specimen to be held under a comparatively high pressure of several tens Pa. Thus, should the first casing be evacuated to a high vacuum as low as 10xe2x88x926 Pa or less which the use of a field emission type electron emitter as the electron beam source requires, creating a difference in pressure in two stages allows not only the first casing to be readily so evacuated but a region of the specimen to be yet left high in pressure for the ease of handling specimens.
The field emission type electron emitter is advantageous in that it emits an electron beam as fine as several hundreds angstroms or less in diameter which is left as still fine as 0.5 xcexcm or less in diameter when incident on a specimen placed at a long focal distance of 150 mm or more.
A high-energy electron diffraction apparatus according to the present invention as set forth preferably provides the said first casing in a region of the said first aperture with an electronic objective lens and objective diaphragm or lens stop for focusing the electron beam from the said source on a specimen, and the said second casing at the said second aperture with a final diaphragm or lens stop.
This construction coupled to the use of a field emission type electron emitter permits the electron beam incident on a specimen to remain as thin as 0.5 xcexcm or less even if the specimen is spaced from the electronic lens at a distance of 150 mm to 300 mm.
Thus, if the electron beam is incident on the specimen at an angle of incidence of, e.g., 3 degrees, its size projected on the specimen being in the order of 100 xcexcmxc3x970.5 xcexcm will be sufficient to identify a thin film structure of 100 xcexcmxc3x97100 xcexcm or less in size such as a pixel of 100 xcexcmxc3x97100 xcexcm or less in size in the so-called combinatorial process.
Also, a high-energy electron diffraction apparatus according to the present invention as set forth preferably includes an axial alignment means disposed between the objective diaphragm or lens stop disposed in the region of the said first aperture in the said first casing and the final diaphragm or lens stop disposed at the said second aperture in the said second casing for adjusting an axial alignment of the electron beam passed through the said objective diaphragm or lens stop.
This construction in which the axial alignment means that may be constituted with axial alignment electrodes or electromagnetic coils allows an electron beam passed through the objective diaphragm or lens stop to be axially aligned and brought into alignment with an axis to a specimen, permits the electron beam to precisely be incident on the specimen.
Further, high-energy electron diffraction apparatus according to the present invention preferably includes, disposed between the said final diaphragm or lens stop disposed at the said second aperture and the said specimen, at least one of an astigmatic correction means for correcting a projected image of the electron beam incident on the said specimen and a scan deflection means for adjustably positioning the electron beam relative to an area on the specimen that is irradiated therewith.
According to this construction, providing for the astigmatic correction means constituted, e.g., by astigmatic correction electrodes causes the electrodes to make the pencil of the electron beam incident obliquely onto a specimen narrower in cross section, thereby making its projected image circular. This permits an electron beam that may if not so corrected be projected to have a size of 100 xcexcmxc3x970.5 xcexcm to be converted into a circular projected image of about 20 xcexcm in diameter, thereby permitting a thin film structure on the specimen to be identified with an equal resolution both lengthwise and breadthwise.
Also, providing for the scan deflection means constituted, e.g., by scan deflection electrodes causes the electrodes to deflect the center of the incident electron beam and to effect its scanning based on a scanning signal. This permits a detector that may be used to detect the impingement of the electron beam on a screen while scanning on the screen to scan in synchronous with a scan-deflecting signal for the scan deflection electrodes, thus to permit observing a magnified image that is composed of a plurality of pixels on the screen, and to make it possible to adjust the position of irradiation with the electron beam with an increased reliability.
Also, a high-energy electron diffraction apparatus according to the present invention preferably includes the said astigmatic correction means and the said scan deflection means as disposed alternately, symmetrically about an axis of the electron beam.
This construction permits scanning an electron beam while rectifying its beam diameter.
Yet further, a high-energy electron diffraction apparatus according to the present invention as set forth preferably includes the said astigmatic correction means and the said scan deflection means as constructed so as to be interchangeable on changing polarity.
According to this construction, simply changing the polarity of a voltage that controls driving the astigmatic correction means and the scan deflection means easily alters their operations.
Also, a high-energy electron diffraction apparatus according to the present invention as set forth preferably uses as or in each of the said astigmatic correction means and the said scan deflecting means a plurality of pairs of electrodes or electromagnetic coils.
This construction, which permits creating an electromagnetic force at a site at which an element of the astigmatic correction means and the scan deflection means, is provided to control the beam diameter of an electron beam and the direction of its scanning.
Still further, a high-energy electron diffraction apparatus according to the present invention preferably uses in the said scan deflection means a first and a second deflecting electromagnetic coil means for permitting the electron beam to be incident on the specimen at any given angle of incidence as desired.
This construction permits a specimen and a selected area thereof to be scanned with an incoming electron beam moving parallel and incident at a given angle adjustable in a wide range, and thus allows, for example, a rocking curve to be promptly obtained.
Also, a high-energy electron diffraction apparatus according to the present invention as set forth preferably includes a screen for projecting thereon the diffracted electron beams from said specimen, disposed at a position spaced apart from the specimen at a distance not greater than 50.
This construction permits electrons reflected by a specimen if a region thereof is placed under a relatively high pressure in the order of several tens Pa to reach the screen without much experiencing collision with and scattering by intervening gaseous molecules, and thus allows the electrons reflected by the specimen to be detected with precision.
Further, a high-energy electron diffraction apparatus according to the present invention as set forth preferably uses as a screen for projecting thereon the diffracted electron beams from said specimen, a porous screen with an electron multiplying function.
This construction permits a weak diffraction signal caused by a small number of electrons reflected to yet yield detecting those electrons from the screen without fail.